[1]}, [2]}
The \(s\) -th order restricted isometry constant (RIC) \(\delta _s\)  of a matrix \(A \in \mathbb {R}^{m \times n}\)  is the smallest number \(\delta _s \ge 0\)  such that
\( \left(1-\delta _{s}\right)\Vert x\Vert _{2}^{2} \le \Vert A x\Vert _{2}^{2} \le \left(1+\delta _{s}\right)\Vert x\Vert _{2}^{2} \) 
